\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=1,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
\draw (0,0)--(2,0)--(4,0);
\ctikzset{inductors/scale=0.75, inductors/width=2.5, inductors/coils=8}  \draw (4,3)--(2,3) to [american inductor] (2,0);
\draw [<->](0,.1)--node[fill=white]{$U_1$}(0,1.5-.1);
\draw [<->](4,.1)--node[fill=white]{$U_2$}(4,3-.1);
\draw [ultra thick](2.3,.1)--node[right]{$n_2$} (2.3,2.9-.1);
\node at (1.7, .7){$n_1$};
\draw (0,1.5)--(2,1.5);
\draw (0,0) [fill=white] circle (1.5pt);
\draw (0,1.5) [fill=white] circle (1.5pt);
\draw (4,0) [fill=white] circle (1.5pt);
\draw (4,3) [fill=white] circle (1.5pt);
\end{circuitikz}
\end{document}